"Crabs" is the common term for lice found in the pubic hair of humans. Crabs is a parasite infection medically known as Pediculosis pubis or pubic lice. Barely the size of a pinhead, lice are organisms that live only with the help of another organism, called a host. There are thousands of types of lice, some of which have developed an attraction to humans. The official name for the organism responsible for pubic lice is Pthirus pubis. Other lice that often infect humans are Pediculus humanus capitis (head lice) and Pediculus humanus corporis (body lice). The term "crabs" seems to come from the microscopic appearance of the pubic louse. The pubic lice organisms are visible to the naked eye in affected areas. The lice are typically seen attached to hair in pubic areas, but may sometimes appear in other areas of the body where coarse hair is present (such as beard, chest, armpits, etc.).

The pubic louse is distinct morphologically (somewhat rounded with three pairs of legs on either side of the body from which it takes its descriptive name) from the head and body louse. The female lifespan is slightly shorter (three weeks), and she produces fewer eggs per day (three) than her counterparts. The eggs attach to the base of the pubic hair shaft for approximately six to eight days before hatching.

A new case study in The New England Journal of Medicine tells a torrid tale of an unnamed 65-year-old man who showed up at a dermatology clinic complaining of an itchy crotch. On examination, the man didn’t appear to have a rash or any lesions, so the doctors investigated his pubic area with a hand-held dermoscope.

The first bit of evidence they uncovered was a nit firmly implanted on a pubic hair. And then came the culprit itself—a freakishly agile, crab-shaped parasite moving from hair to hair like it was nobody’s business.

For the dermatologists, it was a classic case of pubic pediculosis—also known as “crabs.” This happens when the pubic louse, Pthirus pubis, infests a person’s hairy nether regions.

“Pubic pediculosis is usually sexually transmitted but can occur after contact with fomites [materials that are likely to carry infection] such as clothing, bedding, and towels used by an infected person,” write the researchers, who work out of Mexico City’s General Hospital. “The condition most commonly affects teenagers and young adults. This patient reported that he had had no recent sexual contact, and no other sexually transmitted infections were identified on screening.”

The doctors prescribed the oral drug ivermectin, and after two weeks the itching was gone. Which is all fine and well—but what about the psychological scars?